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Directed Hydrozirconation of Propargylic Alcohols

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journal contribution
posted on 10.10.2007 by Donghui Zhang, Joseph M. Ready
Hydrozirconation of terminal alkynes with Cp2ZrH(Cl) (Schwartz reagent) usually proceeds with high selectivity to yield the terminal vinyl zirconium. Under standard hydrozirconation conditions, terminal propargylic alcohols are no exception. However, we report that the sense of regioselectivity is completely reversed when the hydrozirconation is performed on the lithium alkoxide of terminal propargylic alcohols in the presence of ZnCl2. Only the more hindered, branched products are observed. The reaction tolerates silyl and alkyl ethers, aromatic and heteroaromatic rings, and even terminal olefins and alkynes. The intermediate vinyl organometallic reagent can be trapped with I2, enones, and allyl bromide and can participate directly in cross-coupling reactions. Products are isolated in good yields as single regioisomers. We provide evidence that the presence of ZnCl2 alters the kinetic selectivity such that the internal product is favored. More importantly, ZnCl2 inhibits isomerization of the vinyl metal species, likely through transmetalation to a vinyl zinc chloride.